This invention relates to a method for using tropoelastin, and more particularly to a method for producing tropoelastin biomaterials.
Elastic fibers are responsible for the elastic properties of several tissues such as skin and lung, as well as arteries, and are composed of two morphologically distinct components, elastin and microfibrils. Microfibrils make up the quantitatively smaller component of the fibers and play an important role in elastic fiber structure and assembly.
The most abundant component of elastic fibers is elastin. The entropy of relaxation of elastin is responsible for the rubber-like elasticity of elastic fibers. In vertebrates elastin is formed through the secretion and crosslinking of tropoelastin, the 72-kDa biosynthetic precursor to elastin. This is discussed, for example, in an article entitled “Oxidation, Cross-linking, and Insolubilization of Recombinant Crosslinked Tropoelastin by Purified Lysyl Oxidase” by Bedell-Hogan, et al in the Journal of Biological Chemistry, Vol. 268, No. 14, on pages 10345–10350 (1993).
In vascular replacement and repair, the best current option is to implant autologous veins and arteries where the obvious limit is the supply of vessels which can be sacrificed from the tissues they were intended to service. Autologous vein replacements for damaged arteries also tend to be only a temporary measure since they can deteriorate in a few years in high pressure arterial circulation.
When autologous graft material is not available, the surgeon must choose between sacrificing the vessel, and potentially the tissue it sub-served, or replacing the vessel with synthetic materials such as Dacron or Gore-tex. Intravascular compatibility indicate that several “biocompatible polymers”, including Dacron, invoke hyperplastic response, with inflammation particularly at the interface between native tissue and the synthetic implant. Incomplete healing is also due, in part, to a compliance mismatch between currently used synthetic biomaterials and native tissues.
Thirty to forty percent of atherosclerotic stenoses that are opened with balloon angioplasty restenose as a result of ingrowth of medial cells. Smooth muscle ingrowth into the intima appears to be more prevalent in sections of the artery where the internal elastic lamina (IEL) of the artery is ripped, torn, or missing, as in severe dilatation injury from balloon angioplasty, vessel anastomoses, or other vessel trauma that results in tearing or removal of the elastic lamina.
Prosthetic devices, such as vascular stents, have been used with some success to overcome the problems of restenosis or re-narrowing of the vessel wall resulting from ingrowth of muscle cells following injury. However, metal stents or scaffolds being deployed presently in non-surgical catheter based systems to scaffold damaged arteries are inherently thrombogenic and their deployment can result in catastrophic thrombotic closure. Metal stents have also been well demonstrated to induce a significant intimal hyperplastic response within weeks which can result in restenosis or closure of the lumen. Optimal arterial reconstruction would restore the arterial architecture such that normal vascular physiology and biology would be re-established thus minimizing acute and long-term maladaptive mechanisms of vascular homeostasis. Until relatively recently, the primary methods available for securing a prosthetic material to tissue (or tissue to tissue) involved the use of sutures or staples. fibrin glue, a fibrinogen polymer polymerized with thrombin, has also been used (primarily in Europe) as a tissue sealant and hemostatic agent.
Damage to the arterial wall through disease or injury can involve the endothelium, internal elastic lamina, medial smooth muscle and adventitia. In most cases, the endogenous host response can repair and replace the endothelium, the smooth muscle and the adventitial layers over a period of weeks to months depending upon the severity of the damage. However, elastin does not undergo extensive post-developmental remodelling and the capacity for elastin synthesis declines with age. (see “Regulation of Elastin Synthesis in Organ and Cell Culture” by Jeffrey M. Davidson and Gregory C. Sephel in Methods in Enzymology 144 (1987) 214–232. Therefore, once damaged, elastic fibers are not substantially reformed. Neosynthesis of elastin in arterial walls subject to hypertension or neointimal hyperplasia represents the most significant example of post developmental elastin synthesis. This synthesis results in elastic structures mostly composed of elastin fibrils whose organization is unlike normal elastin architecture and probably contributes little to the restoration of normal vascular physiology.
In animal models of intimal hyperplasia or atherosclerosis it is well accepted that disruption of the internal elastic lamina is a prerequisite to reliable production of intimal hyperplasia or atherogenesis in large animals or primates. see Schwartz R. S., et al, in an article entitled “Restenosis After Balloon Angioplasty: Practical Proliferation Model In Porcine Coronary Arteries” in Circulation 1990: 82: 2190–2200. This observation is supported by several lines of evidence that suggest a role for elastin in the biological regulation of several cell types. Pathological studies indicate that elastin provides a secure attachment for endothelial cells and can act as a barrier to macromolecules such as mitogens and growth factors preventing these molecules from entering the media of blood vessels. Lipids, foamy macrophages, and other inflammatory cells do not appear to enter the intima as readily when a substantial and continuous elastin membrane is present immediately to the endothelium according to Sims, F. H., et al, in an article entitled “The Importance of A Substantial Elastic Lamina Subjacent To The Endothelium In Limiting the Progression of Atherosclerotic Changes” in Histopathology (1993) at 23:307–317. In addition, it has been shown by Ooyama, Toshiro and Sakamoto that chemotactic effects of soluble elastin peptides and platelet derived growth factor are inhibited by substratum bound elastin peptides. see “Elastase in the Prevention of Arterial Aging and the Treatment of Atherosclerosis.” see “The Molecular Biology and Pathology of Elastic Tissues” edited by Chadwick, Derek J. and Jamie A. Goode, John Wiley and Sons Ltd, Chichester, England (1995). In vitro experiments show that alpha elastin suppresses the phenotypic transition (contractile to synthetic) of rabbit arterial SMC by interacting with a 130 kDa cell surface elastin binding protein for cell binding sequence VGVAPG. Rabbit smooth muscle cells adhering to elastic fibers appears to favor the contractile over the synthetic state which is identified with restonotic responses to injury. see “Changes in Elastin Binding Proteins During Phenotypic Transition of Rabbit Arterial Smooth Muscle Cells in Primary Culture” by Yamamoto, et al in Experimental Cell Research 218 (1995) pg. 339–345. Similar work by Ooyama and colleagues has demonstrated that the phenotypic change of smooth muscle cells from the contractile to the modified type is significantly retarded when the cells are grown on elastin coated dishes.
Until relatively recently, the primary methods available for securing a prosthetic material to tissue (or tissue to tissue) involved the use of sutures or staples. Fibrin glue, a fibrin polymer polymerized with thrombin, has also been used (primarily in Europe) as a tissue sealant and hemostatic agent.
Laser energy has been shown to be effective in tissue welding arterial incisions, which is thought to occur through thermal melting of fibrin, collagen and other proteins. The use of photosensitizing dyes enhances the selective delivery of the laser energy to the target site and permits the use of lower power laser systems, both of which factors reduce the extent of undesirable thermal trauma.
The present invention combines the advantages of tropoelastin-based products with the advantages of laser welding techniques, and provides a unique method of tissue repair and replacement. The invention makes possible tissue prostheses (particularly, vascular prostheses) that are essentially free of problems associated with prostheses known in the art.
Arterial replacement or reconstruction using tropoelastin based biomaterials not only may provide normal strength and elasticity but also may encourage normal endothelial re-growth, inhibit smooth muscle cell migration and thus restore normal vascular homeostasis to a degree not currently possible with synthetic grafts.
U.S. Pat. No. 4,589,882 is directed to a method for producing synthetic elastomeric polypeptide biomaterial which replicates a portion of the crosslinked tropoelastin polypeptide sequence. This synthetic elastomeric polypeptide biomaterial can be employed in repairing a natural elastic system of an animal body.
U.S. Pat. Nos. 4,721,096 and 4,963,489, which are incorporated herein by reference, disclose a three-dimensional cell culture system in which a living stromal tissue is prepared in vitro by a framework composed of a biocompatible, non-living material formed into a three dimensional structure having interstitial spaces. Collagen has been considered for a biodegradable biomaterials for use as a framework for a three-dimensional, multi-layer cell culture system (see U.S. Pat. No. 4,721,096 and No. 4,963,489).
An improved three-dimensional cell culture systems in which metabolic cycling optimizes the formation of extracellular matrix by cells grown on a three dimensional matrix is disclosed in U.S. Pat. No. 5,478,739 which is herein included as a reference. U.S. Pat. No. 5,478,739 reports production of collagens I, III, and IV, fibronectin, decorin, and non-sulfated glycosaminoglycans by cells in a three dimensional culture.